The invention described in the present disclosure relates to a process and an assembly for the non-destructive inspection of surfaces, particularly for the measurement of small particles, defects, and inhomogeneities on and/or just below the surface of a test object, such particles, defects, and inhomogeneities collectively referred to herein as anomalies. It relates in particular to an instrument described below as the preferred embodiment for inspecting a silicon wafer, the instrument having a light source that generates a light beam, a beam deflector, an optical system that projects the incident beam on a light spot perpendicular to the test object, a photodetector to which the collected light is guided, and an assembly by which the test object is moved by a coordinated translational and rotary movement, so that the light spot scans the whole of the surface along a spiral path.
Such types of process and/or assembly can be used, for example in microelectronics, for the non-destructive checking and inspection of the surfaces of wafers, magnetic storage media, and/or substrates for optical applications, to determine the presence of any particles and/or defects.
The development of wafer-exposure processes has made it possible to manufacture wafer surfaces with ever finer structures parallel to this development, inspection systems that permit the detection of ever more minute defects and particles have become increasingly important. Apart from particles that account for about 75% of all waste in the manufacture of integrated circuits (ICS), inspection systems must be capable of detecting many other types of inhomogeneity, such as variations in the thickness of coatings, crystal defects on and below the surface, etc.
In the final inspection by wafer manufacturers and the inward-goods inspection by chip manufacturers, the unstructured, uncoated wafer must therefore be subjected to extremely searching examination for particle contamination, light-point crystal defects, roughness, polishing scratches, etc. If the test object has a rough surface, then a large amount of stray surface scattering will result. Thus, for this purpose, the test object has a well-polished surface that produces very little diffused light.
In chip manufacture it is usual to monitor each stage of the process, in order to recognize problems as early as possible and thus avoid undue waste. One method of process monitoring is to use so-called monitor wafers which remain unstructured but pass through some of the process stages. Comparison of two measurements, the first before the process stage and the other after it, can thus, for example, help determine the amount of particle contamination due to that process stage or indicate variations in the evenness of the process stage, for example the distribution of the coating thickness over the whole of the wafer. The surfaces subjected to inspection may be rough and metallized, and therefore, produce a great deal of diffused light, or they may be film-coated surfaces that cause interference-fringe effects. Thus, ideally the inspecting instrument has a wide dynamic range to permit defect and particle detection of a wide variety of surfaces.
Prior Art
For the type of inspection described above, so-called laser scanners are particularly suitable. An important feature of these is their high sensitivity to very small defects and the ability to determine the presence of these, and their high throughput. The main differences in the laser scanners now available are the type of scanning they use, their optical configuration, and the manner in which the results are processed.
For applications that require a high throughput and 100% inspection of the whole wafer surface, two processes are mainly used. In the first of these, for example as described in U.S. Pat. No. 4,314,763, the illuminating beam and the collecting optics are stationary, and the test object is scanned spirally by means of a coordinated translational and rotary movement of the test object itself. In the second process, for example as described in U.S. Pat. No. 4,378,159, a rotating or vibrating mirror moves the illuminating beam in one direction linearly back and forth across the wafer, and the whole of the wafer is scanned by virtue of a simultaneous translational movement of the test object perpendicular thereto.
Spiral scanning has the following advantages:
the optical system has no moving parts and thus is simpler;
the illuminated spot and the collector system""s field remain constant during the whole of the measurement procedure, hence the system""s sensitivity is homogeneous over the whole of the test object;
the system takes up less room, because the test object has to be moved only by the length of its radius; and
there is no need to alter the optical system for inspection of bigger objects, only the travel of the translational stage.
The advantages of moving the illuminating beam by means of a mirror or a set of mirrors are:
the test object has to be moved in one direction only, and this is simpler; and
as a rule, scanning is faster.
In the second scanning method, because the illuminating spot moves across the test object and thus the source of diffused light moves in relation to the optical collector system, it cannot ensure an even measuring sensitivity, nor does it permit a rotationally symmetrical arrangement of the collector optics. These are serious drawbacks in laser scanners configured in this manner.
Various optical configurations are known from prior art in the use of a laser scanner for spiral scanning as described above.
For example, U.S. Pat. No. 4,893,932 describes a system which has two differently polarized lasers and two corresponding detectors. The diffused-light intensity of spheres as a function of their diameter has oscillations for diameters within the range of the wavelength used and increases strictly monotonically for smaller diameters. The use of differently polarized light reduces the error in the attribution of diffused-light intensity to particle diameter for the spheres of polystyrene latex (PSL) spheres used for the calibration of laser scanners.
But in practice, the attribution of certain diffused-light intensities to particle diameters depends on so many factors, such as substrate material, films and coatings available, particle material, surface texture of particles, etc., that when the optics and calibration of the equipment are designed only for polystyrene-latex spheres, they tend to make interpretation of the results more difficult. A further major drawback of this method is that the oblique angle of incidence and linear polarization of the laser beam break the symmetry. The measured signal thus depends on the orientation of the defect.
Japanese Patent Application No. 63""14,830 describes collector optics made up of concentric rings, each having six fibre-optic light guides, which are directed to a photomultiplier. The drawbacks of this arrangement are that it fails to cover the central zone near the axis, and the discrete arrangement can achieve rotational symmetry only approximately.
EP-A-0,290,228 describes an arrangement whereby the diffused light is conducted to two detectors. The first detector collects light deflected by about 40 mrad to 100 mrad, the second collects light diffused by more than 100 mrad. Such an angle-resolving method of measurement by means of two detectors makes it possible to classify the defects, but because the collector angle is limited, the system cannot measure very small defects.
DE-A4,134,747 describes a similar solution that uses two detectors designed as arrays, one of which measures the radial and the other the azimuthal light distribution. In this system the test object rotates and the optical system moves linearly.
DD 250,850 also describes an angle-resolving method of measuring diffused light by means of fiber-optic light guides arranged in a circle.
Both the above methods have the drawback that the collector angle is much smaller and closer to specular than that described in the present disclosure.
In this connection, U.S. Pat. No. 4,314,763 describes-a design in which perpendicular incident light and rotational symmetry of the collector optics about the perpendicular of the test object permit measurements regardless of the defect""s orientation. But the lens system used only has a small collector angle and this limits the capability of the design in detection of very small particles at a high throughput rate.
The same inventor""s U.S. Pat. No. 4,598,997 improves the measurement of textured or structured surfaces by the addition of a special mask to the design described above. The purpose of the mask is to suppress the rays deflected by these structures.
A significant drawback of prior art systems is the inability to detect very small surface or near surface defects and particles. With the continual reduction in size of semiconductor structures on wafer surfaces, it is critically important to be able to detect such small anomalies. As shown in table 34 of The National Technology Roadmap for Semiconductors by The Semiconductor Industry Association, 1994, the requirements for defect and particle detection sensitivity will be 0.08 micron in 1998, 0.05 or 0.06 micron in 2001 and down to 0.02 micron in 2010. None of the above referenced systems is capable of achieving sensitivities that are close to such requirements.
As noted above, it is difficult for prior art systems to detect small anomalies such as small particles and defects. Small particles or defects scatter light at large angles to the normal direction of the surface when the surface is illuminated in the normal direction. In normal illumination prior art systems where the light scattered by the surface is collected by a lens system where the axis of the lens system is along the normal direction, the lens system will collect only a small portion of the light scattered by such small anomalies. If large anomalies such as particles or surface defects are also present in addition to the small anomalies, the scattering from such large anomalies will be at much higher intensities compared to and will mask those caused by the small anomalies so that the small anomalies become difficult or impossible to detect. One aspect of the invention is based on the recognition that, since the scattering from the large anomalies is at much higher intensities at specular or near specular collection angles (that is, small angles to the normal) than at large collection angles whereas the scattering from small anomalies have intensities which are more evenly distributed in all directions to the normal, with most of the energy contained in the larger angles. The detection of the small anomalies can therefore be much enhanced by using an ellipsoidal mirrored surface to collect light scattered at relatively large collection angles to the normal and avoiding light scattered at specular or near specular directions.
Thus, one aspect of the invention is directed towards an optical system for detecting contaminates and defects on a test surface comprising a source of light to produce a beam, means for directing the beam along a path onto the test surface, producing an illuminated spot thereon. The system further includes an ellipsoidal mirrored surface having an axis of symmetry substantially coaxial with the path, defining an input aperture positioned proximate to the test surface to receive scattered light therethrough from the surface. The mirrored surface reflects and focuses light that is rotationally symmetric about said axis of symmetry and that passes through the input aperture at an area. The system further includes means for detecting light focused to the area.
When it is known that the surface scattering or haze level is low, and that there are few large defects or point-anomalies, detection sensitivity for small anomalies can be further enhanced by adding to the ellipsoidal mirrored surface of the above apparatus a lens assembly that collects light scattered in a small angle region near the specular direction and focuses the collected light to the same area as the ellipsoidal mirrored surface.
Another aspect of the invention is based on the observation that larger particles scatter light at smaller angles to the normal direction of the surface (i.e. direction of the specularly reflected beam) compared to smaller particles, and the light scattered by the smaller particles is lower in intensity compared to the light scattered by larger particles or defects. Where light scattered in a range of angles covering the collection angles for both large and small particles is collected and directed to a single detector means, and if the detector means is optimized for detecting the low intensity light scattered by smaller particles, the detector means may become saturated by the high intensity light scattered by larger particles. On the other hand, if the detector means is optimized for detecting the high intensity light scattered by larger particles, it is not optimized to detect low intensity light scattered by the smaller particles.
Furthermore, the surface texture itself produces a certain amount of diffracted light in addition to the light scattered by particles. This surface light scatter, commonly referred to as haze, tends to be concentrated at smaller angles near the specularly reflected light beam. If a single detector arrangement is used to detect scattered light from both large and small particles or defects, the effect of haze is to significantly degrade the signal-to-noise ratio for the detection of the smaller defects and particles.
This aspect of the invention is based on the observation that, by collecting scattered light in directions close to and at smaller angles to the specular reflection direction separately from light scattered at larger angles to the specular reflection direction and directing the light scattered at smaller angles to a different detector than the light scattered at larger angles, it is now possible to optimize the two or more detectors separately. Thus, two or more detectors are used: at least a first detector for detecting the low intensity light scattered by smaller particles at larger angles to the specular reflection direction, and at least a second detector for detecting the high intensity light scattered by larger particles at smaller angles to the specular reflection direction. The first detector will not be seriously affected by scattering due to haze, since such scattering decreases rapidly at larger angles from the specular reflection direction.
The above concept is applicable even where the light beam for illuminating the surface to be inspected is at an oblique angle to the surface instead of being perpendicular to the surface and is also applicable for the differentiation, characterization and/or classification of different types of surface or near surface anomalies (referred to below simply as anomalies of surfaces or surface anomalies), including but not limited to anomalies such as scratches, slip lines, crystal originated particles (COPs) as well as contamination particles.
As indicated above, the requirements for detection sensitivity are becoming more and more stringent. For such purpose, it is desirable to focus the illuminating beam onto a small spot on the inspected surface, such as one no larger than 50 microns in dimensions in any direction on the surface. This will enhance signal-to-noise ratio.
Thus, another aspect of the invention is directed towards an apparatus for detecting anomalies of surfaces, comprising means for focusing a light beam along a path towards a spot on a surface, causing a specular reflection, said spot having dimensions less than 50 microns; means for causing rotational and translational movement of the surface, so that the beam scans the surface along a spiral path. The apparatus further comprises a first detector located to detect light scattered by the surface within a first range of collection angles and a second detector located to detect light scattered by the surface within a second range of collection angles, said second range being different from the first range; and an ellipsoidal mirrored surface defining an input aperture positioned approximate to the surface to receive scattered light therethrough from the surface, the mirrored surface reflecting and focusing light passing through the input aperture at the first detector; and a lens assembly collecting light passing through the input aperture, defining collected light, said lens assembly focusing the collected light to the second detector.
Yet another aspect of the invention is based on the observation that if the lens used for collecting light to a detector is also used to focus the illuminating beam towards the surface inspected, stray reflections and scatter of the illuminating beam at the collection lens can cause such background light to be detected by the detector. This introduces errors and is undesirable. Thus, another aspect of the invention is directed towards an apparatus for detecting anomalies of surfaces, comprising means for directing a light beam towards a surface in a direction substantially normal to the surface, said direction defining an axis; means for causing relative motion between the surface and the beam, so that the beam scans the surface; and means for detecting light scattered by said surface. The detecting means includes at least one lens for collecting light to be detected, wherein the directing means directs light towards the surface along an illumination path that does not pass through said at least one lens. The detecting beam preferably includes at least two detectors: a first detector located to detect light scattered by the surface within the first range of collection angles from the axis and a second detector located to detect light scattered by the surface within a second range of collection angles from the axis, said second range being different from the first range.